Frequency scanning receiver

ABSTRACT

A frequency scanning receiver not requiring the time for setting the frequency of the PLL of the first local oscillator and using a method capable of setting the receiving frequency in a simple circuit configuration, wherein the frequency scanning receiver is configured so that in the case where the reference frequency of the PLL of the first local oscillator has been set at a high frequency, when the frequency error is less than the second IF, it is configured so that the frequency error is controlled to be zero by the control circuit switching the frequency of the second local oscillator or the third local oscillator to an appropriate frequency using an output of the frequency discriminator.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a frequency scanning receiver used in the VHF (Very High Frequency) band, UHF (Ultra High Frequency) band or the like, and particularly relates to an improvement of methods of setting a frequency of local carrier wave for converting a frequency.

[0003] 2. Description of the Related Art

[0004] Radio waves used in the VHF band, usually, are allocated so as to be at the intervals of 15 kHz, and in the UHF band, radio waves are allocated so as to be at the intervals of 12.5 kHz. However, although radio waves are allocated at the intervals of 15 kHz, there are a number of groups of radio waves here and there which do not have any multiple relationship with 15 kHz such that these are not evenly allocated at the intervals of 15 kHz over its whole band, there is a group consisted of some radio waves which are allocated at the intervals of 15 kHz, then, there is another group consisted of some radio waves which are allocated at the intervals of 15 kHz but away from the group at the distance of 20 kHz×N. It is also similar circumstances concerning with the frequency band at the intervals of 12.5 kHz. Moreover, resulting from the shortness of radio waves {radio resources or radio propagation paths}, the 15 kHz interval of the VHF band is shifting to 7.5 kHz interval and the 12.5 kHz interval of the UHF band is shifting to 6.25 kHz interval.

[0005] In what is called a full wave scanning receiver for covering radio waves at the intervals over such a wide band, it is a well-worn device that a first local oscillator controlled by a PLL (Phase Locked Loop) for channel selection (tuning) is used. Since a frequency signal of an integer-fold of the reference oscillation frequency by the reference oscillator (crystal oscillator) can be outputted by controlling a dividing ratio of a frequency divider constituting the PLL, a stable scanning first local oscillator can be configured.

[0006] However, in the case where it would have been really applied, the following inconveniences will occur: since in the receiver as configured above, only a frequency signal of an integer-fold of the reference frequency can be outputted, the reference frequency of the first local oscillator is required to set the greatest common divisor of the full receiving frequency (precisely, a frequency as a first local oscillation, but it is similar to a receiving frequency), however, at the present time, radio waves are allocated in miscellaneous arrays of frequency intervals as described above, therefore, it cannot help selecting the reference frequency being extremely low such as 2.5 kHz, 3.125 kHz or the like. If the reference frequency is lowered, the cut-off frequency of the loop filter of the PLL corresponding to this frequency cannot help being also lowered, necessarily, the response of the PLL will be delayed, and it will take a considerable time to set the receiving frequency at the time of channel selection.

SUMMARY OF THE INVENTION

[0007] Hence, an object of the present invention is to provide a frequency scanning receiver capable of setting a receiving frequency at the time of channel selection, not requiring the time for setting the reference frequency of a PLL and in a simple circuit configuration.

[0008] In order to achieve the object, the present invention contrives the point that in a frequency scanning receiver having a first local oscillator using a PLL (Phase Locked Loop), a second local oscillator, a Nth local oscillator (N denotes a positive integer), a frequency discriminator and the like, it is equipped with control means for discriminating the error frequency corresponding to the output of the frequency discriminator at the time of channel selection and controlling so that the output becomes zero by switching the frequency of local carrier wave outputted by the second or the Nth local oscillator.

[0009] In the present invention, the control means may be configured by a memory, setting means for giving frequency setting data of the Phase Locked Loop (PLL) to the memory and setting a frequency of a local carrier wave of the second or Nth local oscillator using an output from the relevant memory, and means for updating setting data of the memory corresponding to the error frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention, wherein:

[0011]FIG. 1 is a diagram of a circuit configuration showing one embodiment of the present invention; and

[0012]FIG. 2 is a diagram of a circuit configuration showing another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Although it is better that the reference frequency of a PLL is as high as possible from the viewpoint of the setting time, if the higher frequency is chosen, since the frequency interval of output carrier wave is widened, by that portion, the number (kinds of errors) of the first IF (first immediate frequency) signal having a frequency error is increased, and if a lower frequency is chosen, on the contrary, the number of the kinds of errors decreases, but the problem of the setting time emerges. Hence, now, supposing that the reference frequency is made on the order of 7.5 kHz or 6.25 kHz, and if a radio wave actually used is channel-selected and received, the number of the kinds of error frequencies is about 3 to about 5. If it is selected so that the frequency of the second local oscillator becomes zero with respect to this first IF signal, corresponding to the error frequency, it is to be a correct frequency in the second IF. It is easily understood how large the value of the error frequency is if the frequency discriminator is provided and the output is observed. Therefore, first, the second local oscillator is set at the normal frequency, receives a radio wave and from the magnitude of the outputted voltage of the frequency discriminator, the error frequencies are discriminated, and it may be made so that the outputted voltage becomes zero by selecting an appropriate frequency from frequencies generated in crystal oscillators for the second local oscillator. If it is such a configuration, the increase of the scale of the circuit is not expected to be so large.

[0014] Although according to the configuration described above, a receiver sufficient for both performance and operation can be configured, if the following circuit is further added to this configuration, still more the operation will be capable of being accelerated. It means that the second local frequency which has been determined by an output of the frequency discriminator as it should be used as well as the first local oscillator frequency (it is possible to use frequency setting data of PLL) has been previously set in a memory such as a rewritable ROM (Read Only Memory) or an EEPROM (Electrically Erasable Programmable ReadOnly Memory). At the time of channel selection, usually, the second local oscillator is set using an output of this ROM, if necessary, the contents of the ROM (setting frequency of the second local oscillator) may be updated using an output of the frequency discriminator. This configuration is capable of being made because the frequency of a radio wave is not frequently changed, according to this configuration, the operation time is shortened by the portion of the response times of the frequency discriminator, determination circuit, switching circuit and the like.

[0015] It should be noted that it is also considered that instead of switching a few pieces of crystal oscillators as second local oscillators, an AFC (Automatic Frequency Control) may be configured by controlling a VCO (Voltage Controlled Oscillator) by an output of the frequency discriminator.

[0016] One embodiment of a frequency scanning receiver of the present invention will be described using the following drawings. FIG. 1 is a diagram showing a circuit configuration of this embodiment. In FIG. 1, the reference numeral 1 denotes an input terminal for a high frequency signal, the reference sign RF denotes a high frequency amplifier, the references sign FC1 denote a first frequency converter, the reference sign LO1 denote first local oscillator, the reference sign REF denote a reference signal generator, the reference numeral 2 denotes a frequency setting signal input terminal, the reference sign IF1 denote a first intermediate cycle amplifier (fist IF amplifier), the reference sign FC2 denote a second frequency converter, the reference sign LO2 denote a second local oscillator (second local oscillator), the reference numeral 3 denotes a switching signal input terminal of the second local oscillator, the reference sign IF2 denote a second intermediate cycle amplifier (second IF amplifier), the reference sign DSCR denote a frequency discriminator, the reference sign DEC denote a determination circuit, the reference sign COL denote a control circuit, and the reference numeral 4 denotes a demodulated signal output terminal.

[0017] A high frequency signal given to the input terminal 1 at the time of channel selection (tuning) is added to the first frequency converter FC1 through the high frequency amplifier RF, and the frequency is converted to the first IF in the first frequency converter FC1 using the local carrier wave outputted from the first local oscillator LO1. The first local oscillator LO1 is controlled by PLL described above and outputs a first local carrier wave of the frequency of an integer-fold of the reference frequency (for example, 7.5 kHz) added by the reference signal generator REF. How many integer-fold the frequency is multiplied by the reference frequency is determined by controlling the frequency dividing ratio of the divider within the PLL using a setting signal added by the terminal 2. The signal converted to the first intermediate frequency (first IF) is added to the second frequency converter FC2 through the first IF amplifier IF1, where it becomes a second IF signal using a second local carrier wave outputted from the second local oscillator LO2. This signal is added to the frequency discriminator DSCR through the second IF amplifier IF2 and demodulated.

[0018] Since initially, the second local oscillator L02 is controlled so that a second local carrier wave of the normal frequency is outputted, in the case where a radio wave having a frequency of an integer-fold of 7.5 kHz (in this case) is received, the output of the frequency discriminator DSCR is zero because there is no frequency error. However, as described above, since there are radio waves having a frequency of +2.5 kHz or −2.5 kHz or the like deviated from the frequency of an integer-fold of 7.5 kHz, when these radio waves are received at the time of channel selection, the frequency discriminator DSCR generates a positive or negative output corresponding to this deviation. Therefore, the determination circuit DEC determines these frequency errors and the control circuit COL controls the frequency error by switching the frequency of the second local oscillator LO2 to an appropriate frequency so that the frequency error (DSCR output) becomes zero.

[0019]FIG. 2 is, as described above in another embodiment of the present invention, a configuration in which a memory MEM is added to the circuit of FIG. 1 and it is made so that it is not necessary to reset the second local oscillator L02 at every time when selecting a channel. Usually, when frequency setting data of the PLL is given from the terminal 2, the rewritable memory MEM makes it as an address input and outputs the second local carrier wave of the frequency previously set, whereby the control circuit COL sets the frequency of the second local oscillator LO2 as an appropriate one. Therefore, when the setting data of the PLL of the first local oscillator L01 is given, since the frequency of the second local oscillator L02 is immediately set through the memory MEM, concerning with the timing, the second local oscillator L02 is set faster than the first local oscillator LO1. Then, the contents of the memory MEM is updated using an output of the determination circuit DEC if necessary. The terminal 5 is the input terminal for a control signal for controlling whether the setting of the frequency of the second local oscillator LO2 is performed using an output of the determination circuit DEC, or it is performed using an output of the memory MEM and controlling whether or not the contents of the memory is updated.

[0020] As described above, all of the frequencies except for frequency of an integer-fold of the reference frequency of the PLL have been described so that these frequencies are coped with the second local oscillator L02, however, needless to say, in the case where the third local oscillator has been provided, these frequencies may be coped with the third local oscillator.

[0021] As described above in detail, in the case where radio waves of the VHF and UHF bands are received and channel-selected by a frequency scanning receiver using a first local oscillator by means of a PLL, the reference frequency of the PLL of the first local oscillator is required to be selected so as to be the greatest common divisor for all of the frequencies within the covering range. However, since all of the actual radio waves are not allocated in equal intervals, it cannot help necessarily selecting lower reference frequencies, as a result of this, it requires the time for setting the PLL. However, according to the present invention, since the PLL can be designed as the frequencies of the received radio waves being allocated at an equal interval, the reference frequency can be set at a higher frequency, and the setting time of the PLL can be made shortened 

What is claimed is:
 1. A frequency scanning receiver having a first local oscillator using a Phase Locked Loop (PLL), a second local oscillator, a Nth local oscillator (N denotes a positive integer), a frequency discriminator and so forth, wherein the frequency scanning receiver comprises control means for determining an error frequency corresponding to an output of the frequency discriminator at a time of channel selection and controlling the output to be zero by switching frequencies of local carrier waves outputted by the second or Nth local oscillator.
 2. A frequency scanning receiver according to claim 1, wherein the control means comprises: a memory, setting means for giving frequency setting data of the Phase Locked Loop (PLL) to the memory and setting a frequency of a local carrier wave of the second or Nth local oscillator using an output of the relevant memory, means for updating setting data of the memory corresponding to the error frequency. 